Effect of order-disorder transition and lattice distortion on the mechanical properties of W-X (X = V, Nb, Ta) solid solutions

With the prosperity of high entropy alloys, short-range ordering is getting more and more attentions and regarded as the deciding role in the hardening or softening of solid solutions. In this work, the phase stability and mechanical properties of short-range ordered and disordered body-centered cubic (BCC) structures of W-X solid solutions (X = V, Nb, and Ta) are systematically investigated through a combination of first-principles calculation, cluster expansion and quasi-harmonic Debye formula. Based on the calculated heat of formation ( Δ H f ), W-X solid solutions show a strong short-range ordering tendency, as those ordered structures have a lower Δ H f than the disordered ones. Generally, short-range ordering results in the decrease of the shear modulus to bulk modulus ratio (G/B) and hardness for W-V and W-Ta systems, while an increase for W-Nb system, manifesting solid-solution softening and hardening effect, respectively. The short-ranged ordered C11 _b W ₂X phase exhibits a singularity in mechanical properties characterized by a higher G/B than solid solution phases compared to other ordered phases. The lattice distortion breaks the symmetric W-W bonds and leads to partial covalency in the W ₂X phases. Besides the increased brittleness, this distortion also causes the increase of ideal strength and the reduction of maximum tensile elongation of W ₂X. The effect of short-range ordering and lattice distortion on the hardening or softening of the W-X systems disappears after the order-disorder transition. The W-V and W-Ta phases have higher order-disorder transition temperature than W-Nb phase. Nevertheless, the transition temperature is relatively low in comparison to their melting points, signifying the important role in controlling the solute concentration at lower temperature.